To overcome the problem of active device switching losses in power converters, while enabling operation at higher switching frequencies, soft-switching converters have been developed. In general, there are two types of soft-switching, or resonant, converters: zero-voltage switching and zero-current switching. Zero-voltage switching involves switching the active devices when there is zero voltage thereacross. Zero-current switching involves switching the active devices when there is zero current therethrough. 1; in commonly assigned U.S. patent application no. 521,061 of R. W. De Doncker and V. Venkataramanan, filed May 3, 1990, now U.S. Pat. No. 5,038,267 which is incorporated by reference herein. The converter of De Doncker and Venkataramanan includes a rectifier for providing rectified ac voltage to a boost dc-to-dc converter, which in turn is coupled to an active clamped resonant dc link (ACRDCL). The ACRDCL employs a bidirectional current switch which can be realized with an active device and an anti-parallel diode. Resonant oscillations of the resonant dc link circuit are maintained via control of an active clamp switch. In order to suitably control and achieve soft-switching of the clamp switch, it is necessary to accurately detect the zero voltage crossings across the switch. This is typically accomplished by using a precision resistive voltage divider and comparator circuitry. Such a voltage divider reduces the maximum voltage that can be measured across the device to levels that are safe for the comparators. A reduction of 50 to 100 or more times is generally necessary. Disadvantageously, therefore, to limit losses in the resistive divider, the voltage detecting circuit has a high input impedance, which results in a high sensitivity to noise signals. Furthermore, the high input impedance of the voltage divider together with any stray capacitance between the input of the comparator and ground create a low pass filter that distorts the voltage signal. As a result, the comparator may turn on the clamp switch at non-zero voltage instants, i.e., in the range from 5 to 20 volts. This causes a hard-switching phenomena which further increases the noise in the system. Hence, it is very difficult to detect accurately the zero-voltage crossings of the clamp switch at higher resonant dc bus voltages and frequencies with a resistive voltage divider.
Accordingly, it is an object of the present invention to provide a new and improved zero-voltage crossing detector which enables accurate and reliable detection of the zero-voltage crossings across an active device to achieve soft-switching thereof.